Mark, a 57-year-old writer, vowed to sculpt a six-pack stomach by New Year's. Tate, a 108-pound freshman at Ashland High School, bet he could reach his goal of doing 20 pullups. He is halfway there.

The challenge started in August with good-natured teasing. Now it has turned into a regimen of punishing workouts for Mark and calisthenics — coupled with plenty of calories — to help Tate pack on 5 pounds of muscle.

"He can't put a pound on if his life depended on it," says Mark. "He's been asking me: 'Can we go have cake and ice cream?' "

The family's "skinny genes" give the advantage to Tate, tasked with lifting his own body weight. Although Mark was a "buffed-out" football player in high school, he says he never had washboard abs, prompting Tate to wager that his dad couldn't get in such good shape if he tried.

"He has a six-pack ab — he's 14 years old," says Mark. "I shot all the way to the moon."

While Mark's aspirations are among the loftiest that personal trainer Mike Sotos has witnessed, he devised a program to maximize Mark's efforts five days per week at Ashland Family YMCA. Promoting core stability, the regimen includes "lots of planks" and body-weight exercises, along with running and lifting weights. Traditional situps and crunches are not part of the routine because they put too much stress on the lower back and can do more harm than good, says Sotos.

"You really don't need to do crunches and situps," he says.

Sotos says that numerous variations on plank, as well as pushups — which exercise the core if done with proper form — are more than sufficient for sculpting the midsection — just maybe not in four months' time.

"It's gonna be hard in the time allotted, but it's possible," says Sotos, owner of Rogue Valley Fitness Training.

Mark has so far managed to trim an inch from his waist and reduce his body fat by 3 percent in the last month. Sotos says consistency with his workout routine has served Mark well. Transforming his body also will require major dietary changes, namely minimizing carbohydrates and other forms of sugar, says Sotos.

"Going out to lunch with Mark now means just a salad," says friend Daric Lucero, who witnessed and is set to judge the bet between father and son.

Showing moral support for Mark's goal has translated into more conscientious choices at local restaurants, says Lucero. The two favor Ashland Food Co-op's deli, where they can take select small servings, as well as healthful options at The Wild Goose, such as oatmeal and fruit instead of bacon and eggs.

Mark already has bragged that he'll be eating bonbons in front of the television after the winner is declared. In the meantime, Tate plots surprise deliveries of chocolate-peanut-butter cream pie to their Ashland home, practices his pullups every other day and plans to save his winnings.

"Mark's battling so much more than just having to go to the gym — age, metabolism."

A struggle against lower back pain is Mark's long-term motivation for getting in shape. He says he hopes that improved core strength will help him ski better this winter and, in months to come, flourish a fly rod and wield a tennis racket with ease.

Formerly a physical therapist, Lucero says his friend is on the right track. And the challenge has inspired Tate, who rarely exercised, to gain strength for pitching baseballs at the high-school level, he adds. Both Lucero and son Ben laud Tate for raising his pullup count from zero to nine in about a month.

"I think it's a great lesson for any of the kids ... you can make huge improvements."